Cathode for electron discharge devices



Patented Aug. 29, 1939 CATHODE FOR ELECTRON DISCHARGE DEVICES Erich Wiegand, Beriin-Waidmannlust, Germany,

assignor to Allgemeine Elelrtricitats Gesellschaft, Berlin, Germany, a corporation of Germany No Drawing. Application June 14, 1935, Serial lln Germany July 11, 1934 5 Claims.

My invention relates to electron discharge devices and more particularly to oxide coated cath odes for such devices.

During operation of thermionic cathodes in electron discharge devices, heating energy losses caused by radiation may be decreased by using for the emitting coatings and the supports therefor materials the radiation from which is small at the operating temperatures.

Particularly suitable for supporting metals are gold, silver, or copper, and alloys thereof. While nickel has a radiation equal to about 15% of a black body, the radiation from. copper amounts to only about 4%. An indirectly heated equipotential cathode of conventional construction comprising a metal sleeve and an electron emissive coating of an alkaline-earth metal oxide, if nickel is used for the sleeve, consumes about four watts of heating energy, while the same cathode with a copper sleeve consumes only two Watts. Cathodes of copper, silver, or gold that is, metals of Group 1 Sub 2 of the Periodic table however, vary in their emissivities, and they are unsatisfactory so far as their actual life is concerned. Oxide cathodes having sleeves of these metals usually lose their electron emissivity after a short period of time. It is believed that the evolution of gases from the metal of the sleeve is responsible though no conclusive evidence to support this theory has so far been found.

It is the principal object of my invention to provide an improved type of oxide coated thermionic cathodes.

Oxide coated cathodes with supporting sleeves consisting of metals of low radiation characteristics and made according to my invention have life and emission characteristics which are substantially the equivalent to cathodes made from nickel or platinum. According to my invention instead of pure metals, such as nickel, I use copper, silver, or gold, or alloys thereof, which apart from the chief metal of lower total radiation, contain a small amount of certain. other metals. For example, particularly favorable results are obtained with sleeves or sheaths which consist of an alloy composed roughly of from 99.9% to 95% copper and of from 0.1 to 5% nickel. A similar action is exhibited also by iron, cobalt, platinum, iridium, osmium, ruthenium, rhodium and palladium, in other Words, all metals belonging to the eighth group of the periodic system of the elements which are capable of being alloyed with copper, silver, and gold.

It is preferable not to choose a mixture such that the favorable radiation properties of the and gold with the other above mentioned metals main metal are lost as a consequence. The radiation characteristic of the alloys of copper, silver differ only slightly from the radiation characteristics of the unalloyed metal as long as the mixture does not exceed a certain limit. This limit is not the same for the various additional metals for a given metal. As a general rule, however, alloys which contain over 10% of the additional metal, are not quite so favorable in their radiation characteristics, although the long life of the cathodes made in accordance with my invention are preserved. V

The cathodes made of alloys of the kind suggested may be made in various ways. As pure metals as are obtainable should be used, for instance, electrolytic copper and nickel made from nickel carbonyl, and the alloy should be made in vacuo or in the presence of gases of a nature which will not be converted with constituents of the alloys. The alloys can be made in the usual way from the pure metals by melting, and may thereupon be worked into ribbons, wires, tubes or sleeves. However, it is also practical to first make ribbons, wires or tubes of the pure main metals and then coat them, say by an electrolytic process, with the added metal, and to finally insure formation of the alloy'in the finished cathode sleeve or core by tempering. It is even practical to cause formation of the alloy after application of the electron emitting coatings and the cathode has been placed in the tube either during or after the exhaustng of the tube, for example, during the forming or activating process. It is suitable to cause the alloying prior to the use of the tube to a point where its characteristics have become stable. In some instances the alloying improves during the life of the tube and the reduction in electron emissivity which is occasionally observed in long life cathodes will not take place because of the increase in working temperature of cathodes made in accordance with my invention.

Using copper as the main metal it may, under certain circumstances, be advantageous to incorporate in the alloy small quantities, up to about 1%, of a de-oxidizing agent such as calcium, silicium, or calcium boride, with the result that such small quantities of oxygen as may have gotten into the alloy during working are made ineffective.

It is not necessary that the entire cathode support should be coated with the added or additional metal. For examle, such portions of the cathode as stay cold in operation may be left unalloyed.

As a general rule, the electron emitting coatings, such as coats of alkali-earth metal oxides, should not be thicker than 100 microns lest the low radiation of the supporting metal wire or sleeve be partly offset by the high radiation of the oxide coating.

With a view to reducing the heat losses it is advantageous in practice to make the current leads from materials having inherently low heat conducting powers.

While the applicant is not to be limited to any particular theory, it is believed that the main metals, after alloying, no longer absorb certain gases which disturb the electron emission or yield them during the operation of the cathode. It may also be that the additional metals tend to increasethe adhesiveness of the electron emitting-coat sr Compared with pure copper, alloys of copper witlrthe metals indicated show also a Mllreater stam towards atmospheric agencies; a is fact isof considerable importance so far as the me s f manufactur ing thermionic cathodes is concerne.

While I have indicated the preferred embodi ments of my invention of which I am now aware and have also indicated only one specific application for which my invention may be employed, it will be apparent that my invention is by no means limited to the exact forms illustrated or the use indicated, but that many variations may be made in the particular structure used and the purpose for which it is employed without departing from the scope of my invention as set forth in the appended claims.

What I claim as new is:

1. In an electron discharge device, a cathode support having an oxide coating and comprising an alloy of only a metal of Group 1 Sub 2 of the periodic table and a metal of the eighth group of the periodic system comprising not more than 10% of the alloy.

2. In an electron discharge device, a cathode support having an oxide coating and comprising an alloy including only copper and a metal of the eighth group of the periodic system of the elements, said metal comprising not more than 10% of the alloy.

3. In an electron discharge device, a thermionic cathode of the oxide coated type including a support for the oxide coating formed of an alloy containing only copper and up to 10% of a metal of the eighth group of the periodic system and up to 1% of a deoxidizing agent.

4. In an electron discharge device, a thermionic cathode of the oxide coated type including a supporting base of only copper of not less than of the total and nickel of not more than ----5% of the total.

5. In an electron discharge device, a thermionic cathode of the oxide coated type including a supporting base of only copper of not less than 95% of the total and nickel of not more than 5% of the total, and a deoxidizing agent up to 1%.

ERICH WIEGAND. 

